Ionic liquids are liquids that are composed entirely of ions. The so-called “low temperature” ionic liquids are generally organic salts with melting points under 100° C., often even lower than room temperature. Ionic liquids may be suitable for example for use as a catalyst and as a solvent in alkylation and polymerization reactions as well as in dimerization, oligomerization, acetylation, metatheses, and copolymerization reactions.
One class of ionic liquids is fused salt compositions, which are molten at low temperature and are useful as catalysts, solvents and electrolytes. Such compositions are mixtures of components which are liquid at temperatures below the individual melting points of the components.
Ionic liquids can be defined as liquids whose make-up is entirely comprised of ions as a combination of cations and anions. The most common ionic liquids are those prepared from organic-based cations and inorganic or organic anions. The most common organic cations are ammonium cations, but phosphonium and sulphonium cations are also frequently used. Ionic liquids of pyridinium and imidazolium are perhaps the most commonly used cations. Anions include, but not limited to, BF4−, PF6−, haloaluminates such as Al2Cl7− and Al2Br7−, [(CF3SO2)2N)]−, alkyl sulphates (RSO3−), carboxylates (RCO2−) and many other. The most catalytically interesting ionic liquids for acid catalysis are those derived from ammonium halides and Lewis acids (such as AlCl3, TiCl4, SnCl4, FeCl3 . . . etc). Chloroaluminate ionic liquids are perhaps the most commonly used ionic liquid catalyst systems for acid-catalyzed reactions.
Examples of such low temperature ionic liquids or molten fused salts are the chloroaluminate salts. Alkyl imidazolium or pyridinium chlorides, for example, can be mixed with aluminum trichloride (AlCl3) to form the fused chloroaluminate salts. The use of the fused salts of 1-alkylpyridinium chloride and aluminum trichloride as electrolytes is discussed in U.S. Pat. No. 4,122,245.
Other examples of ionic liquids and their methods of preparation may also be found in U.S. Pat. Nos. 5,731,101; 6,797,853 and in U.S. Patent Application Publications 2004/0077914 and 2004/0133056.
The alkylation of isobutane with butenes and ethylene in ionic liquids has been described in U.S. Pat. Nos. 5,750,455; 6,028,024; and 6,235,959